The evolution from a society powered by hydrocarbon fuels to one powered by hydrogen requires new systems for hydrogen storage and release where the storage and release system can be reused or recycled. Therefore, hydrogen storage and release is currently being researched as an alternative energy source for fuel cells. This is particularly necessary for those power systems involved with transportation where hydrogen must be contained safely in sufficient quantity to travel a reasonable distance. Given the efficiency of existing fuel cell technology, the typical weight of an average vehicle, and the average distance traveled between refueling stops of a current vehicle, target goals for hydrogen storage systems of 2 kWh/kg (6 weight percent H2) by 2010 and 3 kWh/kg (9 weight percent H2) by 2015 have been set by the U.S. Department of Energy. These targeted capacities are inherently difficult to achieve as liquid hydrogen has a gravimetric capacity of just over 5 weight percent, and the target goals include the mass of the storage tank and balance of plant components for delivery in addition to the mass of the storage medium. The goals have been addressed by various methods of fixing hydrogen.
The fixation of hydrogen by physisorption or by chemisorption is currently under active investigation. These storage systems are generally considered as being “on-board reversible” or “regenerable off-board” depending on whether the material can be refueled with hydrogen while in a vehicle or whether the material must be removed from the vehicle for refueling with hydrogen, respectively. The material must be able to fix hydrogen at a relatively low temperature and pressure at a reasonable rate to be viewed as on-board reversible. A variety of materials, such as metal or complex hydrides, alanates and carbon nanostructures, are being studied for hydrogen storage. A material suitable for hydrogen storage should satisfy three basic requirements: high density storage of hydrogen; stability of stored hydrogen; and release of hydrogen from the material on demand at a relatively low energy input. Appropriate systems for chemisorption have been identified as those with reaction enthalpies of 15 to 75 kJ/mole. Systems with enthalpies significantly below 15 kJ/mole are generally excluded from consideration as being insufficiently irreversible. Materials with reaction enthalpies of 15 to 75 kJ/mole are often not viable for storage systems because of kinetic considerations. Several high density storage materials, such as metal hydrides, require heating at elevated temperatures (>100° C.) for release of hydrogen, making them unattractive for commercial applications. For example, the release of hydrogen by Ca(AlH4)2 has a reaction enthalpy of only 14 kJ/mole but does not release any H2 until temperatures exceed 200° C. No material that shows hydrogen release at temperatures below 100° C. has been identified as a viable candidate.
One approach to a storage system is the absorption of hydrogen on a carbon based absorbent or other nanostructured materials. Such systems have been examined for thermolytic release of hydrogen in a manner that the supporting carbon absorbent can be reused. For example, single-walled carbon nanotubes have been examined but have not been able to achieve the 6 weight percent hydrogen fixation target. The achievement of this level by carbon nanotubes is not anticipated from the data produced to date.
As opposed to the physisorption on carbon, the chemisorption on fullerenes, with or without transition metals, has been examined and can achieve the 6 weight percent target. The hydrogen content of a 1:1 H:C fullerene hydride is 7.7 weight percent and a hydrogen content of 6.3 weight percent has been achieved experimentally by Birch reduction of C60 and 6.1 weight percent has been achieved by direct hydrogenation of C60. To carry out direct hydrogenation of C60, temperatures of at least 400° C. and a pressure of at least 60 MPa is required in spite of the hydrogenation process being exothermic, with an enthalpy of about 60 kJ/mole. The activation energy for uncatalyzed hydrogenation is about 100 kJ/mole. Therefore, the activation energy for the endothermic dehydrogenation of the fullerene hydride is about 160 kJ/mole, which requires even higher temperatures to promote dehydrogenation. The dehydrogenation of crystalline C60H30 requires a temperature of 800° C. to cleanly separate the hydrogen from the intact C60. Additionally, the fullerene hydrides are free of many potentially hazardous properties, such as a spontaneous reactivity with oxygen and/or moisture. Recently, Zhao, Y. et al. (“Hydrogen Storage in Novel Organometallic Buckyballs,” Physical Review Letters 2005, 94, 155504, 1-4) indicates that C60 can potentially store as much as 9 weight percent hydrogen.
Hence, although fullerenes have been identified as promising recyclable hydrogen storage media, an efficient mode for release of the hydrogen at viable release temperatures remains a goal.